The respiratory response to increasing concentrations of endogenous carbon dioxide was measured and the displacement of the respiratory response curve studied in four conditions: (1) the awake subject; (2) the awake subject who was instructed to maintain his respiratory pattern as close to control levels as possible; (3) the hypnotized subject who received no suggestion as to the control of ventilation; (4) the hypnotized subject who received forceful suggestions to try to maintain normal ventilation. The hypnotized subject who received no suggestions had displacement of the Pco2 respiratory response curve 6.9 mm Hg to the right (depression of respiration) at an alveolar ventilation of 15 liters/min. Slope changes were not significant. Suggestion, under hypnosis, displaced the curve 4.0 mm Hg of Pco2. The magnitude of this depression of ventilation is compared to that induced by morphine and normal sleep. Possible mechanisms are discussed.